Tip of the day: (@)...[(R)...] is equal to (@M)...:#... and faster by 18-21%

Tip of the day: (@)...[(R)...] is equal to (@M)...:#... and faster by 18-21%

[psprint]

Hello,
I've discovered that following code:

found=( "${(@M)history:#(#i)*$~search_pattern*}" )

can be replaced by:

found=( "${(@)history[(R)(#i)*$~search_pattern*]}" )

and the speeds are: 215 ms vs. 175 ms, for "(#i)**" final pattern, ${#history[@]} == 89000, and typeset -U found (resolving to 18500 entries). Performing sequence of searches for word "h", "hi", "his", ..., "history" gives accumulated times 1271 ms vs. 1000 ms, difference is ~ 7*40ms, in check. Measured via zprof, by wrapping block of code with () { ... }.

I wonder if there are there any pitfalls in such replacement?

Best regards,

Sebastian Gniazdowski

Re: Tip of the day: (@)...[(R)...] is equal to (@M)...:#... and faster by 18-21%

[Bart Schaefer]

On Oct 21,  8:59am, <psprint@yahoo.com> wrote:
}
} found=( "${(@M)history:#(#i)*$~search_pattern*}" )
} 
} can be replaced by:
} 
} found=( "${(@)history[(R)(#i)*$~search_pattern*]}" )
} 
} I wonder if there are there any pitfalls in such replacement?

There are some differences in the way the pattern may be interpreted
when it appears inside a subscript expression.  There is an entire
section of the documentation (15.2.4 in "info" or HTML docs) devoted
to explaining this.  One paragraph in particular applies here:

    A last detail must be considered when reverse subscripting is
    performed. Parameters appearing in the subscript expression are
    first expanded and then the complete expression is interpreted
    as a pattern. This has two effects: first, parameters behave as
    if GLOB_SUBST were on (and it cannot be turned off); second,
    backslashes are interpreted twice, once when parsing the array
    subscript and again when parsing the pattern. In a reverse
    subscript, it's necessary to use _four_ backslashes to cause a
    single backslash to match literally in the pattern. For complex
    patterns, it is often easiest to assign the desired pattern to a
    parameter and then refer to that parameter in the subscript, because
    then the backslashes, brackets, parentheses, etc., are seen only
    when the complete expression is converted to a pattern. To match
    the value of a parameter literally in a reverse subscript, rather
    than as a pattern, use `${(q)NAME}' to quote the expanded value.

So in your example you don't need "$~search_pattern", the expansion is
always interpreted as a pattern.  I think the literal match advice in
the final sentence could preferentially be ${(b)...} nowadays; there
are a couple of things about the array subscripting doc that might
need update.